In the semiconductor industry, one of the most undesirable processes relates to the means employed for connecting a microcircuit to an associated hybrid circuit device or substrate. For example, a substrate to which a microcircuit must be attached, may be a small single wafer with solder lugs or a large board with places for attaching several microcircuits and/or discrete devices. In either case, the substrate will comprise an insulated support or base with conductive lines applied in a suitable pattern. The lines or leads on the substrate may be applied by vapor deposition, silk screen printing, etching of unmasked metal coating, or other technique, and lead thicknesses will depend upon the method and controls employed in the lead formation process. It has been found that some methods are prone to sever variations in lead thickness.
In more recent state of the art, microcircuits of a type known as beam lead devices are constructed such that they can be packaged or laid directly into the circuits formed on the substrates. Prior to development of beam lead devices, conventional microcircuit devices had to be die attached and included a number of surface pads or terminals each of which had to be individually wire bonded to the respective portions of the substrate circuit. Beam lead devices, however, are provided with thin conductive tabs or leads which project outwardly from the edges of the device and which comprise the terminals which are to be attached to the hybrid circuit for both supporting the device and for electrically interconnecting portions of the device to respective leads on the hybrid circuit.
Bonding a microcircuit device, beam lead or otherwise, to a hybrid circuit by conventional methods and machines usually comprises using gold wire in conjunction with thermal-compression bonding techniques, each end of each wire being required to be separately bonded. This resulted in as many as 32 bonds for a 16-lead device, for example, with the additional processing steps associated with this type of bonding in order to make electrical and mechanical interconnections between the circuit element and the substrate pattern. Each such bond is a potential area of failure due to such mechanisms as misplaced bonds, shorting between bonds, and incomplete welds due to insufficient bonding pressure or too low bonding temperature. Likewise, failure of conventionally bonded devices also sometimes resulted from the use of long loops of gold wire which collapsed during highly accelerated motion, shorting the leads either to the device's patterns or to the substrate patterns. Failure also occurred when the wires sometimes shorted together.
Beam lead devices readily lend themselves to improved bonding techniques since the leads are themselves an integral part of the microcircuit element and provide not only electrical interconnection without separate wires but mechanical strength as well. All of the leads may be bonded in a single operation, are coplanar, and normally sufficient lead separation is inherently provided. In many cases, the microcircuit die is encapsulated in its own hermetic package, with only the leads or beams exposed.
Conventional bonding machines which perform simultaneous multiple bonds are not adapted to efficiently perform this function with beam leads or similar devices. Engagement of a bonding tool which is directed onto the bond area from a direction normal to the bond surface only will sometimes result in failure when sufficient compression is applied to effect pressure on all leads at once. This may be due to uncontrolled lateral flow of metal which occurs, especially when using leads of gold or other soft metal. The resultant deformation of the leads sometimes results in shorting between leads, or may result in complete severing of some leads.
Some bonding machines permit bonding on only one edge of a device at a time. Thus, this considerably lengthens the bonding time sequence. Other bonding machines do not efficiently bond several leads simultaneously when the leads are not truly coplanar, that is, when some leads have surfaces disposed at a higher or lower level than others. Bonding tools in the forms of wheels or discs cannot be employed to overcome this problem because of the miniscule sizes involved.
Some bonding machines having tools which rock, tilt or wobble partially overcome this problem. However, some problem of excessive transverse or sidewise flow of metal can still exist.
In some machines, specially designed optical systems are of necessity employed for use by the operators in aligning a microcircuit device with the circuit pattern on a substrate. Such optical devices employ partially reflecting surfaces so that a tool above a reflecting surface may be visually observed simultaneously with observation of a substrate or of a microcircuit through the reflecting surface. Such devices are sometimes difficult to use and lend to visual problems for an operator. efficient
A flat-ended bonding tool cannot be utilized satisfactorily because of flatness tolerance, and if not specially configured, sometimes causes shearing of beam leads or tearing off of a lead.